The catalyst, as known, is generally selected from compounds of molybdenum or vanadium and (optionally) phosphorus, for instance MoO.sub.3, (NH.sub.4).sub.2 MoO.sub.4, ammonium phosphomolybdate etc. The process can be a dry process, namely in the absence of liquids, as described in U.S. Pat. No. 4,118,391, or in the presence of a hydrophobic organic compound (see UK Patent 1 533 354 and Japanese Patent Publication 81-43 357), which disperses and/or partially dissolves the reaction mixture. The above mentioned techniques permit obtaining high yields, a high purity degree and good pigmentation characteristics, but show some not neglectable drawbacks. In fact, they require high reaction temperatures and long contact times. These drawbacks are very deleterious not only as to the high energy cost, but also as to the negative impact of the high temperature on organic products. In fact, these last degrade although in a low percentage, at temperatures greater than 200.degree. C., giving rise, in some cases, to by-products which change the chromatic tone of the phthalocyanine. Furthermore, the "dry" method requires particular delicate and sophisticated apparatus, especially when a continuous production (requiring a strict control of temperatures and times) is performed. Processes performed at temperatures greater than 180.degree. C. are particularly harmful when the dispersing agent is trichrlorgenzene, as it usually happens on a large scale. In fact, it is known that at high temperatures the reaction mixture (urea+phthalic anhydride+copper or copper compound) causes a degradation of trichlorobenzene, thus forming polychlorodiphenyls, very toxic and difficult to be removed from the finished product.
The Applicants have now found that a particular class of promoters diminish remarkably (and without any drawback) these temperatures and at the same time increase the reaction rate.
In particular, Applicants noted that the reduction of times and temperatures drastically reduces the formation of the degradation products. This is particularly important if the dispersing medium is trichlorobenzene or dichlorobenzene. In fact, in this case, a lowering of the reaction temperatures involves a drastic reduction of polychlorodiphenyls, which originate from the above mentioned solvents. Also Japanese Patent Publication 77-052 927 teaches to reduce the temperature (thus saving energy and reducing polychlorobenzenes) by adding, besides the catalyst, suitable amounts of caustic soda. However, this method is unsatisfactory because of the high ecologic risk connected with the heating of trichlorobenzenes in the presence of caustic soda, which may trigger a strong reaction and consequent uncontrollable production of dioxinic pollution.